The characterization of mutant mice with deafness has led to important discoveries in hearing research, yet the deafness-causing gene defects in a few publicly available mouse strains remain unidentified. One of these strains is Bronx waltzer (bv), which was described in 1979 with respect to the hearing loss and waltzing behavior observed in homozygous mutants. Histological analysis of bv mice revealed a selective loss of cochlear inner hair cells and vestibular hair cells during the perinatal period, without accompanying damage to outer hair cells. Our preliminary studies identified the deafness-causing gene defect in the bv mouse strain. The objective of this application is to functionally characterize the deafness gene of bv mice. Our central hypothesis is that the bv mutation truncates the novel spliceosomal protein Ser/Arg-repetitive matrix 4 (SRRM4) in such a way that this protein fails to mediate key RNA splicing events necessary for the survival of inner hair cells and vestibular hair cells. This hypothesis is based on our preliminary studies which: 1) identified a deletion in the SRRM4 gene of bv mice, 2) rescued the bv phenotype with an SRRM4 transgene, 3) showed that SRRM4 is expressed in hair cells and neurons, 4) revealed that alternative splicing of several mRNAs is aberrant in the bv inner ear and that the affected mRNAs encode functionally related genes, 5) showed that the splicing defects are hair-cell specific, 6) revealed that SRRM4 and its homolog SRRM3 may have redundant functions in certain cell types, 7) identified essential splicing factors that interact with SRRM4, and 8) showed that a nucleotide motif is shared by the pre-mRNAs that are aberrantly spliced in the bv inner ear. We will test our central hypothesis by carrying out 3 specific aims: 1) determine the molecular basis of the hair-cell selectivity of the bv defect, 2) delineate the molecular mechanism by which SRRM4 regulates pre-mRNA splicing, and 3) explore the etiology of hair-cell loss in the bv mice. Under the first aim, we will use knock-out mice to evaluate the extent to which residual SRRM4 function or redundancy between SRRM4 and SRRM3 accounts for the cell-type specificity of the bv defect. Under the second aim, we will use minigenes, RNA-protein crosslinking, immunoprecipitation, and FRET to delineate the molecular mechanism by which SRRM4 regulates alternative splicing. Under the third aim, we will use viral gene delivery in an organ culture setting to determine which of the identified splicing defects underlie the hair-cell loss observed in bv mice. The proposed research is innovative in that it introduces the novel concept that a jointly regulated network of alternative splicing events is required for hearing, and because it employs cutting-edge technologies (exon junction microarrays and new bioinformatics tools) to test the central hypothesis. The proposed study is significant because the identification and characterization of the first deafness gene that regulates essential RNA splicing in the cochlea and vestibular system will lead to the discovery of novel molecular mechanisms, proteins, and protein isoforms that are required for hair cell differentiation and survival.